ECHINODERMATA 



429 



divisions of Echinoderras, which apparently seem to be so 

 clear, the opinions of authors are nevertheless at variance 

 (comp. Semon, No. 55). 



[Through Ludwig's (No. XV.) recent investigations of Cucumaria 

 planci, these conditions have been satisfactorily elucidated. The five 

 invaginations of the water-vascular ring first formed really produce 

 the radial vessels. They are, it is true, at first directed forward, but 

 soon bend backward, thus marking the radii. The tentacular vessels 

 do not arise directly from the ring-canal, but branch off from the radial 

 canals ; however, every radial canal does not give rise to a tentacular 

 vessel, the latter being distributed unsymmetrically to three radial 

 canals only. — K.] 



The internal condi- 

 tions are more evident 

 in Fig. 207, a longi- 

 tudinal section of the 

 pupal stage of Cucn- 

 nnaria (after Selenka). 

 The tentacular evagi- 

 nations are seen coming 

 off from the water- 

 vascular ring forwards, 

 and posteriorly those of 

 the radial vessels. The 

 Polian vesicles also take 

 their origin as evagina- 

 tions of the water-vas- 

 cular ring. In the stasre 

 under consideration the 

 ring is still in connec- 

 tion with the outside 

 world by means of the 

 stone canal and the 

 dorsal pore. This con- 

 nection is afterwards 

 broken, since a cluster 

 of mesenchyma cells 

 subsequently applies 

 itself to the stone canal 



Fio. 207. — Longitudinal section of a larva of 

 Cucumaria doKoIitm, somewhat diagrammatic 

 (after Selbnka). A, anus; Am, ambulacra! 

 (radial) vessels ; D, intestine ; E, enterocceles ; 

 F, feet ; M, mouth ; P, dorsal pore, leading 

 through the stone canal to the water-vascular 

 ring ; T, tentacular vessels ; Wr, water-vascular 

 ring. 



